Cellular communication networks operating according to code division multiple access, CDMA, orthogonal frequency division multiple access, OFDMA, or time division multiple access, TDMA, or combinations thereof, for example, may be endowed with dedicated and non-dedicated channels. Non-dedicated channels may be referred to as common channels. For example downlink common channels are transmitted by a base station and are receivable by all terminals in the cell. Examples of downlink common channels include physical downlink control channel, PDCCH, and physical broadcast channel, PBCH, of the long term evolution, LTE, cellular system.
To improve reliability of communication in cellular systems, communication channels thereof are transmitted in coded form, which may be referred to as channel coding. Channel coding may comprise convolutional coding, block coding or turbo coding, for example. When a payload frame is channel coded, the length of the frame is increased by so-called redundancy information. The ratio of the number of payload bits to the coded frame length may be referred to as code rate k/n. This means that for every k bits of payload, the coder generates n bits of data. For example a rate of ⅓ may indicate that one third of the bits of a coded frame are payload bits and two thirds are redundancy bits. ½ may indicate that half of the bits are payload and half are redundancy bits. Redundancy bits enable receivers to detect and, in some cases, correct errors that occur in transmission.
Having a low code rate means that there are more redundancy bits relative to the number of payload bits, when compared to a high code rate. This means that the coded data is more robust against transmission errors, but transmitting the payload bits requires more resources since many redundancy bits need to be transmitted to ensure the payload is successfully communicated. Having a high code rate on the other hand inserts fewer redundancy bits, which means that whereas the payload bits require fewer resources for transmission, the data is less robustly protected from corruption en route. Selecting the code rate therefore presents a trade-off between efficient usage of resources on the one hand, and reliability on the other.